SEASONAL EVOLUTION OF UPPER OCEAN THERMAL STRUCTURE BETWEEN TASMANIA AND ANTARCTICA

We describe the upper ocean thermal structure between Tasmania and Ant arctica based on thirteen repeat temperature sections occupied between 1991 and 1994. The sections cross three main fronts. The subtropical front is found between Tasmania and the South Tasman Rise in each of t he sections. The subantarctic front (SAF) is composed of two parts, wh ich have distinct thermohaline signatures and behave somewhat independ ently: the northern part, associated with the 6-8 degrees C isotherms, is characterised by large meridional gradients of both temperature an d salinity; the southern part is associated with a weaker meridional t emperature gradient and negligible salinity gradient between the 3 deg rees and 5 degrees C isotherms. The northern part of the SAF is locate d between 50 degrees S and 51 degrees S in each of the sections, but t he position of the southern part of the SAF is more variable with time . A cold core eddy or meander is found north of the SAF throughout the 1993-1994 austral summer. The polar front (PF) is found near 53 degre es S in all sections. Dynamic height is estimated for each of the XBT sections by exploiting the tight correlation in this region between ve rtically-integrated temperature and dynamic height. Dynamic height dec reases relatively smoothly with latitude between 50 degrees S and 53 d egrees S, so that the SAF, PF and the water between the two fronts for ms a broad belt of eastward flow relative to a deeper level. The diffe rence in dynamic height at the sea surface relative to 2000 m is 1.03 dyn m between 47 degrees S and 60 degrees S and is constant through th e 1993-1994 austral summer to within the accuracy of the method (rms e rror approximate to 0.07 dyn m). The dynamic height expression of the cold core eddy reaches a maximum of 0.23 dyn m in February 1994. The u pper 100 m of the water column warms by about 1.6 degrees C between De cember and March south of 54 degrees S, corresponding to an average wa rming rate of 95 W m(-2). Changes in heat content at other latitudes a re dominated by meridional shifts of the fronts, and no clear seasonal trend can be identified. (C) 1997 Elsevier Science Ltd.